.\" iostat manual page - (C) 1998-2020 Sebastien Godard (sysstat <at> orange.fr)
.TH IOSTAT 1 "SEPTEMBER 2021" Linux "Linux User's Manual" \-*\- nroff \-*\-
.SH NAME
iostat \- Report Central Processing Unit (CPU) statistics and input/output
statistics for devices and partitions.

.SH SYNOPSIS
.ie 'yes'no' \{
.B iostat [ \-c ] [ \-d ] [ \-h ] [ \-k | \-m ] [ \-N ] [ \-s ] [ \-t ] [ \-V ] [ \-x ] [ \-y ] [ \-z ]
.BI "[ \-\-compact ] [ \-\-dec={ 0 | 1 | 2 } ] [ { \-f | +f } " "directory" " ] [ \-j { ID | LABEL | PATH | UUID | ... } ] "
.BI "[ \-o JSON ] [ [ \-H ] \-g " "group_name " "] [ \-\-human ] [ \-\-pretty ] [ \-p [ " "device" "[,...] | ALL ] ] ["
.IB "device " "[...] | ALL ] [ \-\-debuginfo ] [ " "interval " "[ " "count " "] ] "
.\}
.el \{
.B iostat [ \-c ] [ \-d ] [ \-h ] [ \-k | \-m ] [ \-N ] [ \-s ] [ \-t ] [ \-V ] [ \-x ] [ \-y ] [ \-z ]
.BI "[ \-\-compact ] [ \-\-dec={ 0 | 1 | 2 } ] [ { \-f | +f } " "directory" " ] [ \-j { ID | LABEL | PATH | UUID | ... } ] "
.BI "[ \-o JSON ] [ [ \-H ] \-g " "group_name " "] [ \-\-human ] [ \-\-pretty ] [ \-p [ " "device" "[,...] | ALL ] ] ["
.IB "device " "[...] | ALL ] [ " "interval " "[ " "count " "] ]"
.\}

.SH DESCRIPTION
.RB "The " "iostat"
command is used for monitoring system input/output device
loading by observing the time the devices are active in relation
to their average transfer rates. The
.B iostat
command generates reports
that can be used to change system configuration to better balance
the input/output load between physical disks.
.PP
The first report generated by the
.B iostat
command provides statistics
concerning the time since the system was booted, unless the
.B \-y
option is used (in this case, this first report is omitted).
Each subsequent report
covers the time since the previous report. All statistics are reported
each time the
.B iostat
command is run. The report consists of a
CPU header row followed by a row of
CPU statistics. On
multiprocessor systems, CPU statistics are calculated system\-wide
as averages among all processors. A device header row is displayed
followed by a line of statistics for each device that is configured.
.PP
The
.I interval
parameter specifies the amount of time in seconds between
each report. The
.IR "count " "parameter can be specified in conjunction with the " "interval"
.RI "parameter. If the " "count " "parameter is specified, the value of " "count"
.RI "determines the number of reports generated at " "interval " "seconds apart. If the"
.IR "interval " "parameter is specified without the " "count " "parameter, the"
.B iostat
command generates reports continuously.

.SH REPORTS
The
.B iostat
command generates two types of reports, the CPU
Utilization report and the Device Utilization report.

.IP "CPU Utilization Report"
The first report generated by the
.B iostat
command is the CPU Utilization Report. For multiprocessor systems, the CPU values are
global averages among all processors.
The report has the following format:
.RS
.IP %user
Show the percentage of CPU utilization that occurred while
executing at the user level (application).
.IP %nice
Show the percentage of CPU utilization that occurred while
executing at the user level with nice priority.
.IP %system
Show the percentage of CPU utilization that occurred while
executing at the system level (kernel).
.IP %iowait
Show the percentage of time that the CPU or CPUs were idle during which
the system had an outstanding disk I/O request.
.IP %steal
Show the percentage of time spent in involuntary wait by the virtual CPU
or CPUs while the hypervisor was servicing another virtual processor.
.IP %idle
Show the percentage of time that the CPU or CPUs were idle and the system
did not have an outstanding disk I/O request.
.RE
.PP
.IP "Device Utilization Report"
The second report generated by the
.B iostat
command is the Device Utilization Report.
The device report provides statistics on a per physical device
or partition basis. Block devices and partitions for which statistics are
to be displayed may be entered on the command line.
If no device nor partition is entered, then statistics are displayed
for every device used by the system, and
providing that the kernel maintains statistics for it.
If the
.B ALL
keyword is given on the command line, then statistics are
displayed for every device defined by the system, including those
that have never been used.
Transfer rates are shown in 1K blocks by default, unless the environment
variable
.B POSIXLY_CORRECT
is set, in which case 512\-byte blocks are used.
The report may show the following fields, depending on the flags used (e.g.
.BR "\-x" ", " "\-s " "and " "\-k " "or " "\-m" "):"
.RS
.IP Device:
This column gives the device (or partition) name as listed in the
.IR "/dev " "directory."
.IP tps
Indicate the number of transfers per second that were issued
to the device. A transfer is an I/O request to the
device. Multiple logical requests can be combined into a single I/O
request to the device. A transfer is of indeterminate size.
.IP "Blk_read/s (kB_read/s, MB_read/s)"
Indicate the amount of data read from the device expressed in a number of
blocks (kilobytes, megabytes) per second. Blocks are equivalent to sectors
and therefore have a size of 512 bytes.
.IP "Blk_wrtn/s (kB_wrtn/s, MB_wrtn/s)"
Indicate the amount of data written to the device expressed in a number of
blocks (kilobytes, megabytes) per second.
.IP "Blk_dscd/s (kB_dscd/s, MB_dscd/s)"
Indicate the amount of data discarded for the device expressed in a number of
blocks (kilobytes, megabytes) per second.
.IP "Blk_w+d/s (kB_w+d/s, MB_w+d/s)"
Indicate the amount of data written to or discarded for the device expressed
in a number of blocks (kilobytes, megabytes) per second.
.IP "Blk_read (kB_read, MB_read)"
The total number of blocks (kilobytes, megabytes) read.
.IP "Blk_wrtn (kB_wrtn, MB_wrtn)"
The total number of blocks (kilobytes, megabytes) written.
.IP "Blk_dscd (kB_dscd, MB_dscd)"
The total number of blocks (kilobytes, megabytes) discarded.
.IP "Blk_w+d (kB_w+d, MB_w+d)"
The total number of blocks (kilobytes, megabytes) written or discarded.
.IP r/s
The number (after merges) of read requests completed per second for the device.
.IP w/s
The number (after merges) of write requests completed per second for the device.
.IP d/s
The number (after merges) of discard requests completed per second for the device.
.IP f/s
The number (after merges) of flush requests completed per second for the device.
This counts flush requests executed by disks. Flush requests are not tracked for partitions.
Before being merged, flush operations are counted as writes.
.IP "sec/s (kB/s, MB/s)"
The number of sectors (kilobytes, megabytes) read from, written to or
discarded for the device per second.
.IP "rsec/s (rkB/s, rMB/s)"
The number of sectors (kilobytes, megabytes) read from the device per second.
.IP "wsec/s (wkB/s, wMB/s)"
The number of sectors (kilobytes, megabytes) written to the device per second.
.IP "dsec/s (dkB/s, dMB/s)"
The number of sectors (kilobytes, megabytes) discarded for the device per second.
.IP rqm/s
The number of I/O requests merged per second that were queued to the device.
.IP rrqm/s
The number of read requests merged per second that were queued to the device.
.IP wrqm/s
The number of write requests merged per second that were queued to the device.
.IP drqm/s
The number of discard requests merged per second that were queued to the device.
.IP %rrqm
The percentage of read requests merged together before being sent to the device.
.IP %wrqm
The percentage of write requests merged together before being sent to the device.
.IP %drqm
The percentage of discard requests merged together before being sent to the device.
.IP areq\-sz
The average size (in kilobytes) of the I/O requests that were issued to the device.
.br
Note: In previous versions, this field was known as avgrq\-sz and was expressed in sectors.
.IP rareq\-sz
The average size (in kilobytes) of the read requests that were issued to the device.
.IP wareq\-sz
The average size (in kilobytes) of the write requests that were issued to the device.
.IP dareq\-sz
The average size (in kilobytes) of the discard requests that were issued to the device.
.IP await
The average time (in milliseconds) for I/O requests issued to the device
to be served. This includes the time spent by the requests in queue and
the time spent servicing them.
.IP r_await
The average time (in milliseconds) for read requests issued to the device
to be served. This includes the time spent by the requests in queue and
the time spent servicing them.
.IP w_await
The average time (in milliseconds) for write requests issued to the device
to be served. This includes the time spent by the requests in queue and
the time spent servicing them.
.IP d_await
The average time (in milliseconds) for discard requests issued to the device
to be served. This includes the time spent by the requests in queue and
the time spent servicing them.
.IP f_await
The average time (in milliseconds) for flush requests issued to the device
to be served.
The block layer combines flush requests and executes at most one at a time.
Thus flush operations could be twice as long: Wait for current flush request,
then execute it, then wait for the next one.
.IP aqu\-sz
The average queue length of the requests that were issued to the device.
.br
Note: In previous versions, this field was known as avgqu\-sz.
.IP %util
Percentage of elapsed time during which I/O requests were issued to the device
(bandwidth utilization for the device). Device saturation occurs when this
value is close to 100% for devices serving requests serially.
But for devices serving requests in parallel, such as RAID arrays and
modern SSDs, this number does not reflect their performance limits.
.RE

.SH OPTIONS
.TP
.B \-c
Display the CPU utilization report.
.TP
.B \-\-compact
Don't break the Device Utilization Report into sub\-reports so that all the metrics get displayed
on a single line.
.TP
.B \-d
Display the device utilization report.
.if 'yes'no' \{
.TP
.B \-\-debuginfo
Print debug output to stderr.
.\}
.TP
.B \-\-dec={ 0 | 1 | 2 }
Specify the number of decimal places to use (0 to 2, default value is 2).
.TP
.BI "\-f " "directory"
.RE
.BI "+f " "directory"
.RS
Specify an alternative directory for
.B iostat
to read devices statistics. Option
.BR "\-f " "tells " "iostat " "to use only the files located in the alternative directory, "
whereas option
.B +f
tells it to use both the standard kernel files and the files located in the alternative directory
to read device statistics.

.IR "directory" " is a directory containing files with statistics for devices managed in userspace."
It may contain:

\- a "diskstats" file whose format is compliant with that located in "/proc",
.br
\- statistics for individual devices contained in files whose format is compliant with that of files located in
"/sys".

In particular, the following files located in
.I "directory"
.RB "may be used by " "iostat" ":"

.IR "directory" "/block/" "device" "/stat"
.br
.IR "directory" "/block/" "device" "/" "partition" "/stat"

.IR "partition" " files must have an entry in " "directory" "/dev/block/ directory, e.g.:"

.IR "directory" "/dev/block/" "major" ":" "minor" " \-\-> ../../block/" "device" "/" "partition"
.RE
.TP
.BI "\-g " "group_name " "{ " "device " "[...] | ALL }"
Display statistics for a group of devices.
The
.B iostat
command reports statistics for each individual device in the list
then a line of global statistics for the group displayed as
.I group_name
and made up of all the devices in the list. The
.B ALL
keyword means that all the block devices defined by the system shall be
included in the group.
.TP
.B \-H
This option must be used with option
.B \-g
and indicates that only global
statistics for the group are to be displayed, and not statistics for
individual devices in the group.
.TP
.B \-h
This option is equivalent to specifying
.BR "\-\-human \-\-pretty" "."
.TP
.B \-\-human
Print sizes in human readable format (e.g. 1.0k, 1.2M, etc.)
The units displayed with this option supersede any other default units (e.g.
kilobytes, sectors...) associated with the metrics.
.TP
.BI "\-j { ID | LABEL | PATH | UUID | ... } [ " "device " "[...] | ALL ]"
Display persistent device names. Keywords
.BR "ID" ", " "LABEL" ", "
etc. specify the type of the persistent name. These keywords are not limited,
only prerequisite is that directory with required persistent names is present in
.IR "/dev/disk" "."
Optionally, multiple devices can be specified in the chosen persistent name type.
Because persistent device names are usually long, option
.B \-\-pretty
is implicitly set with this option.
.TP
.B \-k
Display statistics in kilobytes per second.
.TP
.B \-m
Display statistics in megabytes per second.
.TP
.B \-N
Display the registered device mapper names for any device mapper devices.
Useful for viewing LVM2 statistics.
.TP
.B \-o JSON
Display the statistics in JSON (JavaScript Object Notation) format.
JSON output field order is undefined, and new fields may be added
in the future.
.TP
.BI "\-p [ { " "device" "[,...] | ALL } ]"
Display statistics for
block devices and all their partitions that are used by the system.
If a device name is entered on the command line, then statistics for it
and all its partitions are displayed. Last, the
.B ALL
keyword indicates that statistics have to be displayed for all the block
devices and partitions defined by the system, including those that have
never been used. If option
.B \-j
is defined before this option, devices entered on the command line can be
specified with the chosen persistent name type.
.TP
.B \-\-pretty
Make the Device Utilization Report easier to read by a human.
The device name will be printed on the right side. The report may also be broken
into sub\-reports if there are many metrics to display (use
.B \-\-compact
option to prevent this).
.TP
.B \-s
Display a short (narrow) version of the report that should fit in 80
characters wide screens.
.TP
.B \-t
Print the time for each report displayed. The timestamp format may depend
on the value of the
.BR "S_TIME_FORMAT " "environment variable (see below)."
.TP
.B \-V
Print version number then exit.
.TP
.B \-x
Display extended statistics.
.TP
.B \-y
Omit first report with statistics since system boot, if displaying
multiple records at given interval.
.TP
.B \-z
Tell
.B iostat
to omit output for any devices for which there was no activity
during the sample period.

.SH ENVIRONMENT
The
.B iostat
command takes into account the following environment variables:
.TP
.B POSIXLY_CORRECT
When this variable is set, transfer rates are shown in 512\-byte blocks instead
of the default 1K blocks.
.TP
.B S_COLORS
By default statistics are displayed in color when the output is connected to a terminal.
Use this variable to change the settings. Possible values for this variable are
.IR "never" ", " "always " "or " "auto"
(the latter is equivalent to the default settings).
.br
Please note that the color (being red, yellow, or some other color) used to display a value
is not indicative of any kind of issue simply because of the color. It only indicates different
ranges of values.
.TP
.B S_COLORS_SGR
Specify the colors and other attributes used to display statistics on the terminal.
Its value is a colon\-separated list of capabilities that defaults to
.BR "H=31;1:I=32;22:M=35;1:N=34;1:Z=34;22" "."
Supported capabilities are:
.RS
.TP
.B H=
SGR (Select Graphic Rendition) substring for percentage values greater than or equal to 75%.
.TP
.B I=
SGR substring for device names.
.TP
.B M=
SGR substring for percentage values in the range from 50% to 75%.
.TP
.B N=
SGR substring for non\-zero statistics values.
.TP
.B Z=
SGR substring for zero values.
.RE
.TP
.B S_TIME_FORMAT
If this variable exists and its value is
.B ISO
then the current locale will be ignored when printing the date in the report
header. The
.B iostat
command will use the ISO 8601 format (YYYY\-MM\-DD) instead.
The timestamp displayed with option
.B \-t
will also be compliant with ISO 8601 format.

.SH EXAMPLES
.TP
.B iostat
Display a single history since boot report for all CPU and Devices.
.TP
.B iostat \-d 2
Display a continuous device report at two second intervals.
.TP
.B iostat \-d 2 6
Display six reports at two second intervals for all devices.
.TP
.B iostat \-x sda sdb 2 6
Display six reports of extended statistics at two second intervals for devices
sda and sdb.
.TP
.B iostat \-p sda 2 6
Display six reports at two second intervals for device sda and all its
partitions (sda1, etc.)

.SH BUGS
.IR "/proc " "filesystem must be mounted for"
.BR "iostat " "to work."
.PP
Kernels older than 2.6.x are no longer supported.
.PP
.RB "Although " "iostat"
speaks of kilobytes (kB), megabytes (MB)..., it actually uses kibibytes (kiB), mebibytes (MiB)...
A kibibyte is equal to 1024 bytes, and a mebibyte is equal to 1024 kibibytes.

.SH FILES
.IR "/proc/stat " "contains system statistics."
.br
.IR "/proc/uptime " "contains system uptime."
.br
.IR "/proc/diskstats " "contains disks statistics."
.br
.IR "/sys " "contains statistics for block devices."
.br
.IR "/proc/self/mountstats " "contains statistics for network filesystems."
.br
.IR "/dev/disk " "contains persistent device names."

.SH AUTHOR
Sebastien Godard (sysstat <at> orange.fr)

.SH SEE ALSO
.BR "sar" "(1), " "pidstat" "(1), " "mpstat" "(1), " "vmstat" "(8), " "tapestat" "(1), " "nfsiostat" "(1),"
.BR "cifsiostat" "(1)"
.PP
.I https://github.com/sysstat/sysstat
.br
.I http://pagesperso\-orange.fr/sebastien.godard/
